1. Field
The present invention relates to an apparatus, system, and method of inspecting an image formed by an image forming apparatus, and a recording medium storing an image inspection control program.
2. Background
The recent image forming system may be provided with an inspection apparatus, which reads a printed image output from an image forming apparatus, generates a master image from image data of the printed image, and compares the read image with the master image to determine whether the printer image sufficiently reproduces the image data. To compare between the read image and the master image, each pixel in the read image needs to be compared with a corresponding pixel in the master image. In view of this, the pixel position of the master image may be corrected so as to match the pixel position of the read image, which may shrink in size or may be skewed during print processing, by enlarging or reducing the size of the master image or rotating the master image. For example, the inspection apparatus may specify a plurality of reference points in the read image, which correspond to a plurality of reference points in the master image, by pattern matching. Based on the positional shifts of the reference points between the read image and the master image, a correction parameter to be used for correcting the master image may be set.
While the reference point is usually defined as a marker in case of offset printing, when cut paper is used for printing, the reference point needs to be defined to make comparison between the read image and the master image. Japanese Patent Application Publication No. 2004-195878 discloses a technique of adding a yellow dot pattern to an image to be formed on a recording sheet, and using the yellow dot pattern as the reference point to detect the positional shift in the printed image with the master image. This technique of using the yellow dot pattern is applicable when the printed image is a color image that is formed using a color image forming apparatus. When a monochrome image forming apparatus is used or when a monochrome image is to be printed, the yellow dot pattern is not added such that it would not be possible to inspect the printed image using the reference point.
Further, when the yellow dot pattern is added to an area of the printed image having yellowish color, it may be difficult for the inspection apparatus to extract the yellow dot pattern from the read image. In such case, Japanese Patent Application Publication No. 2004-195878 uses a projection histogram generated for each one of the vertical and horizontal directions of the image. However, when the printed image is skewed, such technique using the histogram cannot be used to correctly detect the positional shift in the printed image.